Polysaccharide derivatives, e.g. cellulose ethers, can be successfully used as thickeners and water retention additives in hydraulic setting compositions, e.g. gypsum spray plaster compositions. The complete wetting of e.g. gypsum after the addition of water is delayed to some extent due to the high surface energy of the gypsum particles even if no polysaccharide derivatives are present. In the presence of polysaccharide derivatives, however, the water which wets all gypsum particles is thickened as the polysaccharide particles are solubilized. As a consequence, the mobile aqueous phase is less effective in wetting the gypsum. During the application unwetted gypsum lumps become visible in the plaster as it gets spread evenly on a wall after spraying. These lumps are optical defects and additional work and force is required during the subsequent working steps of plaster application to remove them completely, if ever possible. A hydraulic setting composition, especially a gypsum spray plaster, with no or minimal lumps would be desirable as it allows easier and faster application. The workers can apply more area per day of defect free hydraulic setting composition.
One way of effective lump reduction in gypsum plaster would be the use of granular cellulose ethers (<50 wt. % having a particle size of up to 200 microns). However, the high content of large cellulose ether particles would result in poor water retention of the sprayed plaster on the wall. During the short time span between mixing and spraying (about 10 sec) the coarse particles will not completely dissolve and the plaster will have insufficient water retention and post-thickening will be observed.
Another way of reducing lumps in hydraulic setting compositions, e.g. gypsum plaster, would be the use of Methocel™ J-grades of cellulose ethers (Dow Chemical, Midland, Mich.). The high amount of substituents give the cellulose ether some thermoplastic properties so that it almost completely loses its fibrous structure and shows similar fibre thickness and diameter of the equivalent particle circle as the polysaccharide derivatives of the present invention. Unfortunately, the production costs of such cellulose ethers are very high and the process yields a waste water stream that is both undesirable and costly to handle and/or dispose.
GB 2262527A describes the pre-treatment of a wet cellulose ether filter cake prior to milling in order to obtain a cellulose ether with reduced lump formation. The wet filter cake has a water content of 30-80 wt. % and is cooled down to a temperature of 40-120° C. The examples show that the process can increase the bulk density of the cellulose ethers. No reference is given to the grinding technology used in the process. And apart from a mean particle size no information is given on the particle size distribution and on the morphology of the cellulose ether powder.
The problem addressed by the invention is to provide a hydraulic setting composition, e.g. a mortar or plaster composition, having an improved lump rating or an improved water retention behaviour, or both. A further problem addressed by the invention is to provide a method of making a hydraulic setting composition, wherein one or more of its properties selected from lump rating and water retention can be adjusted by simply including a tailored particulate polysaccharide compound to the hydraulic setting composition, which imparts the desired properties to the hydraulic setting composition.